In recent years, electronic information equipment, especially various portable types of electronic information equipment, have become remarkably widespread. Most types of electronic information equipment use batteries as power sources and include built-in power converters such as DC-DC converters. In general, a power converter is constructed as a hybrid module in which individual parts of active components, such as switching elements, rectifiers and control ICs, and passive elements, such as inductors, transformers, capacitors and resistors, are located on a ceramic board or a printed board of plastic or similar material. In recent years, the miniaturization of inductors has been an issue in miniaturization of power converters.
An inductor generally includes wire wound around a core of ferrite material. Power inductors operate as energy-storage devices that store energy in a magnetic field during the power supply's switching-cycle on time and deliver that energy to a load during off time. There are different types of power inductors, including discrete wire-wound inductors, discrete surface-mount (SMD) inductors, discrete non-wire wound (e.g., solenoid type) inductors and discrete multi-layer inductors. Wire-wound inductors may be based on round wire or flat wires, wound around a ferrite core, with encapsulation. Examples of wire-wound inductors include those made by TOKO. Discrete SMD inductors include wire wound around a magnetic core with the resulting structure being coated with a resin. Taiyo-Yuden's inductors are examples of surface-mount inductors.
“Open Spools” are often used to enable the winding of the wire conductors which form inductor coils. However, winding wire is not the most efficient process to form a toroidal coil. Typical toroidal coil inductors require “feeding” of the wire through a center hole in a doughnut shaped ferrite core, which is a complex process to automate.
Multilayer inductors include multiple layers of ferrite, each with a pattern of conductive material (Ag for example) that forms part of the inductor coils. The ferrite layers are stacked and conductive vias between adjacent layers connect the patterned conductors to form the coils.
U.S. Pat. No. 6,930,584 discloses a microminiature power converter including a semiconductor substrate on which a semiconductor integrated circuit is formed, a thin film magnetic induction element, and a capacitor. The thin film magnetic induction element includes a magnetic insulating substrate, which may be a ferrite substrate, and a solenoid coil conductor in which a first set of conductors is formed on a first principal plane of the magnetic insulating substrate, a second set of conductors is formed on a second principal plane of the magnetic insulating substrate, a set of conductive connections is formed in through holes passing through the magnetic insulating substrate providing electrical connection between the first and second set of conductors and forming the inductor coils, and a set of conductive connections formed in through holes passing through the magnetic insulating substrate providing electrodes electrically connected through the through hole. A surface of the coil conductor may be covered with an insulating film or a resin in which magnetic fine particles are dispersed. However, the thickness of the inductor coil conductors is limited to the thickness of the conductive layer deposited on the magnetic insulating substrate.
U.S. Pat. No. 6,630,881 discloses a multi-layered chip inductor including coil-shaped internal conductors formed inside a green ceramic laminate. Each of the coil-shaped internal conductors spirals around an axial line in the laminating direction of the green ceramic laminate. An external electrode paste is applied onto at least one laminating-direction surface of the green ceramic laminate, which external electrode paste connects to an end of the coil-shaped internal conductor. The green ceramic laminate is cut along the laminating direction into chip-shaped-green ceramic laminates each having the coil-shaped internal conductor inside.
U.S. Pat. No. 4,543,553 discloses a chip-type inductor comprising a laminated structure of a plurality of magnetic layers in which linear conductive patterns extending between the respective magnetic layers are connected successively in a form similar to a coil so as to produce an inductance component. The conductive patterns formed on the upper surfaces of the magnetic layers and the conductive patterns formed on the lower surfaces of the magnetic layers are connected with each other in the interfaces of the magnetic layers and are also connected to each other via through-holes formed in the magnetic layers, so that the conductive patterns are continuously connected in a form similar to a coil.
U.S. Pat. No. 7,046,114 discloses a laminated inductor including ceramic sheets provided with spiral coil conductor patterns of one turn, ceramic sheets provided with spiral coil conductor patterns of two turns, and ceramic sheets provided with lead-out conductor patterns, which are laminated together. The coil conductor patterns are successively electrically connected in series in regular order through via holes. The via holes are disposed at fixed locations in the ceramic sheets.
U.S. Pat. No. 5,032,815 discloses a lamination type inductor having a plurality of ferrite sheets assembled one above the other and laminated together. The uppermost and lowermost sheets are end sheets having lead-out conductor patterns facing each other. A plurality of intermediate ferrite sheet each has a conductor pattern on one surface which corresponds to a 0.25 turn of an inductor coil and a conductor pattern on the other surface which corresponds to a 0.5 turn of an inductor coil. Each ferrite sheet has an opening through which the conductor patterns of the 0.25 and 0.5 turn are electrically connected to form a 0.75 turn of an inductor coil on each ferrite sheet. The conductor patterns on the successive intermediate sheets are connected to each other for forming an inductor coil having a number of turns, which is a multiple of 0.75, and the conductor patterns on the upper surface of the uppermost of the plurality of intermediate ferrite sheets and the lower surface of the lowermost of the intermediate ferrite sheets are electrically connected to the conductor patterns on the surfaces of the end sheets for forming a complete inductor coil.
U.S. patent application Ser. No. 12/011,489 of Alpha & Omega Semiconductor LTD discloses an inductor comprising a toroid magnetic core with lead frame conductors having low resistance, but not planar since lead frames are placed on top and bottom of the magnetic core substrate
Many conventional power inductors are not planar, have relatively high resistance due to the limited thickness (size) of the inductor conductors, do not have a completely closed magnetic loop or do not incorporate a means of connecting other components in a stacked configuration (which minimizes the overall area).
It would be desirable to develop a power inductor structure which maximizes the inductance per unit area and minimizes resistance by using low-resistivity conductor and appropriate assembly techniques, in combination with the lowest number of turns, and small physical size.
It would be further desirable to produce a device that enables small foot print and thin outline with high-volumes and a low-cost of manufacture.
It is within this context that embodiments of the present invention arise.